[#3789] Destroy mutable objects only if needed (#3799)

* [#3789] Don't observe objects if they are already mutable objects. Add some null checks.
* Don't destroy mutable in Selection.js if it wasn't created in that context.
* Remove * listeners and add null checks
* Don't delete _observers and _globalEventEmitters on $destroy. Pop all items off the _observers list for a mutable domain object.

Co-authored-by: Andrew Henry <akhenry@gmail.com>

src/api/objects/MutableDomainObject.js

@@ -112,9 +112,11 @@ class MutableDomainObject {
         return () => this._instanceEventEmitter.off(event, callback);
     }
     $destroy() {
-        this._observers.forEach(observer => observer());
-        delete this._globalEventEmitter;
-        delete this._observers;
+        while (this._observers.length > 0) {
+            const observer = this._observers.pop();
+            observer();
+        }
+
         this._instanceEventEmitter.emit('$_destroy');
     }